In an electric vehicle and a hybrid electric vehicle, for example, in order to supply power to an electric motor for driving these vehicles, or in order to store electronic energy by charging power from the electric motor for using energy generated during braking as a generator or from a power source installed on the ground, a rechargeable battery (a secondary battery) is used.
In this case, in order to maintain an optimum battery status for a long time, it is necessary to monitor a battery status, a state of charge (SOC) in particular, at all times as battery management.
As conventional SOC detection methods, there are known a current integration method (also called as a Coulomb counting method or a book keeping method) and an open circuit voltage method. According to the current integration method, data about the voltage and the current of the battery therein/thereout are all recorded in a chronological order, and an electric charge at the present time is obtained by time integration of the current by using the data, and thus the SOC is obtained by using an initial value of the electric charge of the battery and a full charge capacity. According to the open circuit voltage method, an input current value and a terminal voltage value of the battery are input and, by using a battery equivalent circuit model, an open circuit voltage value serving as a model state quantity is sequentially estimated, thereby estimating the SOC from the open circuit voltage value.
There are advantages and disadvantages to each method. That is, the current integration method of the former, although being capable of estimating the SOC in a short time more accurately than the open circuit voltage method of the latter for estimating the SOC by using the open circuit voltage value, the current integration method requires monitoring at all times and, due to accumulation of errors over time, an accuracy deteriorates. On the other hand, although the open circuit voltage method of the latter does not require the monitoring at all times, due to a small fluctuation of the open circuit voltage associated with a change in the SOC, the open circuit voltage method is no better than the current integration method of the former in estimating a variation quantity of the SOC in a short time.
As such, there has conventionally known a method that, by correcting an estimation error of the SOC obtained by the above methods, attempts to improve the accuracy of the estimation of the SOC.
As one of conventional SOC estimating apparatuses employing such a method, there is known a SOC estimation apparatus including: a first electrical quantity calculating means for calculating a SOC from an open circuit voltage estimation value estimated by employing the open circuit voltage method using an adaptive digital filter based on a discharge current and a terminal voltage of the battery; a second electrical quantity calculating means for carrying out time integration of a charge current and the discharge current of the battery by employing the current integration method and, based on thus obtained integrated value, calculating a changing quantity of the electricity charged in the battery; and an offset quantity estimating means for estimating, from a difference between the changing quantities obtained by the first electrical quantity calculating means and the second electrical quantity calculating means, an offset quantity serving as a measured value error of a discharge current measuring instrument, wherein correction for the offset quantity is carried out to reduce the measured value error of the discharge current measuring instrument and to improve the accuracy of a measured current value, thereby improving the accuracy of the estimation of an internal state of the battery such as the SOC (for example, see Patent Document 1).
Also, as another conventional SOC estimation method, there is known a method including: a battery information obtainment step of measuring battery information (the current value, a voltage value, and temperature); a current correction step of calculating a current integrated value by carrying out integration of a corrected current value corrected by using the voltage value; an integrated capacity calculation step of calculating a current integrated value by carrying out integration of the corrected current value and also calculating a current integrated capacity reflecting the current integrated value and a battery charge and discharge efficiency; a correction determination step of determining, based on a battery forward voltage capacity calculated from the battery information, whether to correct the current integrated capacity; and an integration correction step of obtaining a remaining capacity of the battery with or without correcting the current integrated capacity according to the determination, thereby attempting to improve accuracy of the calculation of the SOC by correcting a measurement error of the battery information (for example, see Patent Document 2).